1. Field of the Invention
The present invention relates to a semiconductor laser device having a high characteristic temperature and, more particularly, to a semiconductor laser device having a high characteristic temperature and operating with a lower threshold current density at a room temperature.
2. Description of the Related Art
A semiconductor laser device having an emission wavelength of 1.3 μm band and operating with a small optical dispersion within an optical fiber is expected to be used as a light source in the subscriber's optical device in an optical communication system.
As a typical example of such a semiconductor laser device having an emission wavelength of 1.3 μm, a GaInAsP-based semiconductor laser device formed on an InP substrate is being developed. In this respect, a GaInAsP-based semiconductor laser device operating with a threshold current as low as 1 mA or less at a room temperature is expected for practical use. However, this semiconductor laser device has a poor characteristic temperature of the threshold as low as 50K to 70K, and accordingly, it is necessary that this semiconductor laser device be cooled by using a cooling device for a stable operation thereof.
Although there are strong requests that a laser transmission module having a built-in semiconductor laser device be reduced in size and have a lower power dissipation, the cooling device for improving the characteristic temperature of the laser device does not meet these requests. Thus, it is desired to develop a long-wavelength semiconductor laser device having an improved characteristic temperature and without the need for the cooling device.
Patent Publication JP-A-2000-277867 describes a semiconductor laser device formed on a GaAs substrate and having an improved characteristic temperature around 180K. This semiconductor laser device has a quantum well (QW) active layer structure including a GaInNAs well layer having a nitrogen content of 3% or less and a pair of AlGaInAsPN or GaAsP barrier layers having a negative strain with respect to the GaAs substrate. It is reported that the samples of this semiconductor laser device exhibited characteristic temperatures as high as around 130K to 200K in the experiments.
FIG. 1 schematically shows a typical layer structure for the conventional GaInNAs-based semiconductor laser device. The semiconductor laser device, generally designated by numeral 10, includes an n-type GaAs (n-GaAs) substrate 12, and a layer structure epitaxially grown on the (100) lattice plane of the n-GaAs substrate 12. The layer structure includes, consecutively as viewed from the bottom, a 0.2-μm-thick n-GaAs buffer layer 14, a 1.5-μm-thick n-InGaP cladding layer 16, a 0.1-μm-thick GaAs barrier layer 18, a 8-nm-thick GaInNAs(Sb) well layer 20, a 0.1-μm-thick GaAs barrier layer 22, a 1.5-μm-thick p-InGaP cladding layer 24 and a p-GaAs cap layer 26.
In the conventional semiconductor laser device lasing at a wavelength of 1300 nm band, the GaxIn1-xN1-yAsy well layer constituting the laser active layer has an In content (1-x) of 0.3 to 0.4, and a nitrogen content (1-y) of 0.005 to 0.02. A minute amount of Sb may be added to the GaInNAs well layer for improvement of the crystallinity thereof by an amount of, for example, 0.3 to 2% with respect to the amount of the V-group atoms to obtain a GaInNAsSb well layer. This is expressed by the formula GaInNAs(Sb) designating the above well layer in this text. The barrier layer is generally implemented by a GaAs layer.
For further improvement of the crystallinity of the active layer structure, the well layer and the barrier layers are subjected to a thermal treatment at a high temperature between 500 and 850 degrees C., after the epitaxial growth using a MOCVD or gas source MBE technique. This high-temperature thermal treatment increases the photoluminescence (PL) intensity of the epitaxial layers by several times compared to the PL intensity of the as-grown epitaxial layers, i.e., the epitaxial layers before the thermal treatment. The high-temperature thermal treatment causes the bandgap of the GaInNAs(Sb) well layer to be changed to a shorter emission wavelength.
The GaInNAs-based semiconductor laser device reported heretofore has a problem in that the threshold current density thereof at a room temperature is higher than the threshold current density of a GaInAsP-based semiconductor laser device formed on an InP substrate.